c# - 获取前 x 个相等元素的索引

Question

我需要在排序的整数列表中获取所有彼此相等的最大元素的索引。

所以给出这个列表

elements:   {1 , 1, 2, 3, 4, 4, 5, 6, 7, 7, 8, 9, 10, 11, 11, 12, 13, 13, 13}
index:       0   1  2  3  4  5  6  7  8  9  10 11 12  13  14  15  16  17  18
                                                                  ^   ^   ^

我会得到这个输出

{16,17,18}

到目前为止我有

list.Select((x, i) => new {x, i})

要获取索引，但我不能使用OrderBy()orFirst()因为Single()我需要所有最大元素的索引，而不仅仅是最上面的索引。

有没有一种优雅的方式来实现这一点（使用 LINQ 或其他方式）？

score 7 · Accepted Answer

因此项目被排序，您只需要获取具有最大值的第一个项目的索引（该项目将与最后一个项目具有完全相同的值），然后创建从该索引开始到列表末尾的索引范围：

var items = new List<int> {1,1,2,3,4,4,5,6,7,7,8,9,10,11,11,12,13,13,13};
int startIndex = items.IndexOf(items[items.Count - 1]);
var indexes = Enumerable.Range(startIndex, items.Count - startIndex);

score 0 · Accepted Answer

我不会使用 LINQ，因为它是对集合的简单 foreach。

//var data = new[] {1, 1, 13, 2, 3, 4, 4, 5, 6, 7, 7, 8, 9, 10, 11, 11, 12, 13, 13, 13};
var data = new[] {1, 1, 2, 3, 4, 4, 5, 6, 7, 7, 8, 9, 10, 11, 11, 12, 13, 13, 13};

var largest = int.MinValue;
var indices = new List<int>();

foreach (var x in data.Select((value, idx) => new {value, idx}))
{
    if (x.value > largest)
    {
        indices.Clear();
        largest = x.value;
    }

    // if unsorted
    //if (x.value == largest) indices.Add(x.idx);

    // if sorted you don't need to check against largest
    indices.Add(x.idx);
}

Console.WriteLine("largest = {0}; indices = {1}", largest, string.Join(", ", indices));

虽然如果你必须使用 LINQ，你可以使用这个选项而不是foreach：

data.Select((value, idx) => new {value, idx})
    .Aggregate(indices, (idxs, n) =>
    {
        if (n.value > largest)
        {
            idxs.Clear();
            largest = n.value;
        }

        //unsorted
        if (n.value == largest) idxs.Add(n.idx);

        //sorted
        //idxs.Add(n.idx);
        return idxs;
    });

score 0 · Accepted Answer

这将为您提供所有具有重复索引的元素的结果：

var result = elements.Select((value, index) => new { value, index })
             .Where(g => elements.FindAll(v => v == g.value).Count > 1)
             .GroupBy((a) => a.value).OrderByDescending((g) => g.Key).Take(3);
            //I placed Take(3) as example since you said you need to find
            //elements who are equal to each other,so only those that are
            // not distinct(have duplicates) get into the collection.

            //this will loop through the results and display the value(the item
            //on the list) and its respective index.
            foreach (var item in result.SelectMany(g => g))
            {
                string outcome = item.value + " - " + item.index;
                Console.WriteLine(outcome);
            }

score 0 · Accepted Answer

简单/懒惰的方式：

        var a = new[] {1, 1, 2, 3, 4, 4, 5, 6, 7, 7, 8, 9, 10, 11, 11, 12, 13, 13, 13};
        var b = new List<int>();
        var max = a.Max();
        for (var i = 0; i < a.Length; i++)
        {
            if (a[i] == max) b.Add(i);
        }

c# - 获取前 x 个相等元素的索引

4 回答 4

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